ntpサーバー

$ sudo apt-get install ntp
$ sudo vi /etc/ntp.conf

# more information.
server ntp.nict.jp
server ntp.nict.jp
server ntp.nict.jp

$ sudo /etc/init.d/ntp restart

しばらくして，

$ sudo ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp-b3.nict.go. .NICT.           1 u  119  256  377   41.039   -5.353  10.750
+ntp-a2.nict.go. .NICT.           1 u  135  256  337   44.075   -0.237  15.912
+europium.canoni 193.79.237.14    2 u   20  256  377  285.417   -8.406  18.196